It is perhaps useful to remember that there are no civilians within the Borg collective when discussing this. I'm not sure that it is genocide. Would Picard hesitate to use an antibiotic to kill a virus? Probably not, and just because Borgs were once-upon-a-time individuals and happen to be mainly humanoid in form, does not mean we should see them as anything other than a multi-cellular virus.

__________________One day soon, man is going to be able to harness incredible energies, energies that could ultimately hurl us to other worlds in... some sort of spaceship.